The present invention relates to a technique of precharging data lines before sampling image signals into the data lines.
Display panels for performing a display using electro-optical variation of an electro-optical material, such as display panels using liquid crystal, can be classified into several kinds depending upon driving types thereof. However, active matrix display panels for driving pixel electrodes with three-terminal switching elements have approximately the following structure. That is, in this kind of liquid crystal panel, liquid crystal is interposed between a pair of substrates, and one substrate is provided with a plurality of scanning lines and a plurality of data lines to intersect each other. In addition, as shown in FIG. 10, pairs of a thin film transistor (hereinafter, referred to as “TFT”) 116 and a pixel electrode 118 are provided to correspond to respective intersections between the scanning lines 112 and the data lines 114. The other substrate is provided with a transparent counter electrode (common electrode) 108 opposite to the pixel electrodes and the counter electrode is kept at a constant voltage LCcom. Furthermore, the respective opposite surfaces of both substrates are provided with an alignment film having been subjected to a rubbing process such that a major-axis direction of liquid crystal molecules is continuously twisted, for example, by about 90° between both substrates, while the respective rear-surface sides of both substrates are provided with a polarizer corresponding to the alignment direction, respectively.
For the purpose of convenient explanation, it is supposed that the total number of scanning lines 112 is “m” and the total number of data lines 114 is “6n” (m and n are integers, respectively). Then, the pixels are arranged in a matrix shape of m rows×6n columns correspondingly to the intersections between the scanning lines 112 and the data lines 114.
In order to reduce and prevent leakage of electric charges in a liquid crystal capacitor, a storage capacitor 119 is provided at each pixel. One end of the storage capacitor 119 is connected to the pixel electrode 118 (the drain of the TFT 116) and the other end thereof is grounded in common through a capacitor line 175.
Light passing between the pixel electrodes 118 and the counter electrode 108 is optically rotated by about 90° depending upon degrees of twist of liquid crystal molecules when an effective voltage value of the liquid crystal capacitor is zero, while the liquid crystal molecules are inclined in an electric-field direction as the effective voltage value is increased, so that the optical rotation disappears. For this reason, for example, in a transmissive liquid crystal panel, in a case of a normally-white mode where polarizers of which polarizing axes are perpendicular to each other correspondingly to an alignment direction are disposed at the incident side and the rear-surface side, respectively, the light is transmitted when the effective voltage value of the liquid crystal capacitor is zero, so that a white color is displayed (the transmittance becomes large). As the effective voltage value is increased, the quantity of transmitted light is decreased, so that a black color is displayed (the transmittance is minimized). Therefore, by applying image signals with voltages corresponding to grayscale levels (or brightness) of pixels to the pixel electrodes 118 via the data lines 114 and controlling the effective voltage value of the liquid crystal capacitor in units of a pixel, a predetermined display is possible.
Since the liquid crystal capacitor is basically AC-driven, the image signals applied to the pixel electrodes 118 have a voltage range shown in FIG. 11 and alternately take a high-potential side voltage and a low-potential side voltage about a reference voltage Vc which is a about of amplitude. Here, the writing when the voltages applied to the pixel electrodes 118 become higher than the voltage Vc is referred to as a positive writing and the writing when the voltages applied to the pixel electrodes 118 become lower than the voltage Vc is referred to as a negative writing. The reference voltage Vc may be considered as the voltage LCcom of the counter electrode 108, but may be slightly different depending upon characteristics of the TFT 116.
Here, supposed that the ground voltage which is the low potential side of a source voltage is 0V and the high potential side is 14V, the image signal allowing a pixel to display a black color which is the lowest grayscale level in the negative writing have, for example, 2V. Similarly, the image signals allowing a pixel to display a white color which is the highest grayscale level in the negative writing, to display a white color in the positive writing, and to display a black color in the positive writing have 6V, 8V, and 12V, respectively, and the reference voltage is 7V. These voltage values are determined for the purpose of convenience.
This kind of liquid crystal panel has a problem that display quality is deteriorated due to a so-called vertical cross-talk. The vertical cross-talk is a phenomenon that for example, as shown in FIG. 12, when a black area is intended to be displayed in a window against a gray background having the same grayscale level, gray areas adjacent to the top and bottom of the black area become brighter than other gray areas.
For the purpose of convenient explanation, in FIG. 12, a display area 100a is divided into areas A, B, and C in the horizontal scan direction and is also divided into areas D, E, and F in the vertical scan direction. Total nine areas are specified by an area of the horizontal scan direction and an area of the vertical scan direction. For example, the black area which is displayed in a window can be marked by (B-E).
The major cause for the vertical cross-talk is leakage of light from the TFT 116 interposed between the pixel electrodes 118 ad the data lines 114. Specifically describing the leakage of light, the gate-source voltage VDS of a TFT and the drain current ID generally have a characteristic relation indicated by a solid line in FIG. 13. Since poly silicon constituting a TFT has a light-transmitting property, a black matrix is provided such that light is not incident to a channel portion of the TFT. However, since it is difficult to completely intercept the light, the characteristic is shifted to left as indicated by a dotted line. Even when the characteristic is shifted, the drain current ID little flows if the source (data line) voltage is still lower than the gate (scanning line) voltage, but the drain current ID flows if the source voltage is slightly lower than the gate voltage. That is, the off resistance is decreased.
Here, in a case where performing the display shown in FIG. 12, when the scanning lines belonging to the area B are selected and the voltage (2V) corresponding to a positive black color is sampled into the data lines belonging to the area E, the voltages of the scanning lines belonging to the areas A and C are not selected, so that the low-potential voltage of the source voltage is 0V. Accordingly, since the gate voltage is slightly lower than the source voltage in the TFTs belonging to the gray areas (A-E) and (C-E), the off resistances thereof are decreased in the TFTs of the above areas and the voltages of the pixel electrodes 118 become close to the voltage of the counter electrode, so that the effective voltage value applied to the liquid crystal capacitor is decreased.
On the contrary, since the voltage corresponding to a negative black color is never sampled into the data lines belonging to the areas D and F, the off resistances of the TFTs belonging to the gray areas (A-D), (B-D), (C-D), (A-F), (B-F), and (C-F) are not decreased. As a result, the effective voltage value applied to the liquid crystal capacitor is not decreased as much.
Therefore, in a normally-white mode, the pixels of the gray areas (A-E) and (C-E) are brighter than the pixels of the gray areas (A-D), (B-D), (C-D), (A-F), (B-F), and (C-F) due to decrease in the effective voltage value.
On the other hand, since a parasitic capacitor exists in the respective data lines, the time required for sampling the image signals into the data lines is elongated, and since the voltages of the image signals sampled right before remain in the data lines, the voltages of the data lines (pixel electrodes) when the image signals are subsequently sampled are changed. In order to prevent these problems, there has been known a technology of precharging the data lines with a constant voltage.
Here, as the voltage for precharging the data lines, a voltage (9V) corresponding to a positive gray color is preferably considered for performing the positive writing and a voltage (5V) corresponding to a negative gray is preferably considered for performing the negative writing. The reason is that in the characteristic relation (V-T characteristic) between the effective voltage value applied to the liquid crystal capacitor and the transmittance thereof, the variation of the transmittance relative to the effective voltage value is maximized when the gray color is displayed (when the transmittance is 50%). The reason is also that by precharging the data lines to the voltage (5V or 9V) corresponding to a gray color in advance, the image signals of the voltage corresponding to a gray color can be sampled into the data lines with a high speed and an intermediate grayscale level can be accurately displayed.
As the voltage for precharging the respective data lines in this way, the voltage corresponding to a gray color is preferably considered, but in order to make the vertical cross-talk invisible, there has been also suggested a technology of applying the voltage (2V) corresponding to a black color as the precharge voltage before performing the negative writing.
In this way, by precharging the voltage corresponding to a black color before performing the negative writing, in the TFTs of the gray areas (A-D), (B-D), (C-D), (A-F), (B-F), and (C-F), the gate voltage is also lower by 2V than the source voltage and thus the off resistance is decreased, similarly to the TFTs belonging to the gray areas (B-F) and (C-F). For this reason, the gray areas (A-D), (B-D), (C-D), (A-F), (B-F), and (C-F) become bright due to decrease in the effective voltage value applied to the liquid crystal capacitor, similarly to the gray areas (B-F) and (C-F). As a result, difference in grayscale level between the gray areas disappears, so that the vertical cross-talk is invisible.
Considering that the voltage (2V) corresponding to a black color is used as the precharge voltage of the negative writing in order to make the vertical cross-talk invisible, an ideal gray color is displayed at the positive writing by using the voltage corresponding to a white color, that is, an amplitude-about voltage as needed, as the precharge voltage of the positive writing.